The existing SMS protocol, as shown in FIG. 1, is conceptually composed of four levels, Teleservice, Transport, Relay and Link. The Teleservice level supports basic SMS functions with sub-parameters of Short Message Data Parameter (SMDP); the Transport level forms terminal points between the Service Message Center (SMC) and the terminal at the message transmission/reception phase, and confirms the status of communication readiness; the Relay level establishes the transmission channel in accordance with the status of the terminal and the length of the message; and the Link level adds information needed by the IS-95-A[or TDMA] protocol to the data and send the message to the SMC.
FIG. 2 shows the structure of existing SMS protocol parameters by level. The SMS parameter for CHARi message on the Link level is capsulated, and the parameters are analyzed and sent to the upper level. On the basis of Data Burst Message on IS-95-A[or TDMA] which functions as the Link level, and through Um Interface on the Relay level, the Transport level is formed. Transport level messages include Teleservice Identifier which is an identifier for voicemail and normal messages, Originating Address, Bearer Reply Option, and Bearer Data.
Bearer Data forms the Teleservice level, and the Teleservice level provides application services. The Teleservice level comprises User Data which includes the content of the received message, Message Center Time Stamp, Priority Indicator, Privacy Indicator, Alert on Deliver, and Language Indicator.
SMS messages are not limited to mobile-phone-to-mobile-phone communication, but may also be transmitted from a computer. If a computer is used, the message is sent to a Message Center (MC) through the Internet and then the MC sends the text message to mobile phones, and the message can be sent to a number of mobile terminals in a short period of time.
However, as SMS formed on the basis of Data Burst Message (Link level) of IS-95-A[or TDMA], the size of the message may not exceed 200 bytes. Excluding the parameter information, the maximum size of a message is generally 160 bytes, and in Korea the maximum size is regulated at 160 bytes. Thus, a message that exceeds 160 bytes is sent in two or more messages, and the receiving mobile phone displays the message in two or more messages as well.
Due to the 160 byte limitation on data size in SMS, Multimedia Message Service (MMS) has been developed and used. However, while MMS enables transmission of voice, picture, and video data as well as unlimited-length text messages, the billing for MMS includes per-packet data usage charges in contrast to SMS per-message billing, and both the sender and receiver incur service charges. Commercial MMS messages are particularly unwelcome.